The present invention relates to an electrolytic treatment apparatus suitable for providing a rough surface on a metal web by using an AC current, and particularly relates to an electrolytic treatment apparatus for producing a printing-plate support constituted by a rough-surfaced aluminum plate to be used as a offset printing plate.
Aluminum plates have been used as printing-plate supports, particularly, as planographic printing plate supports. Such aluminum plates have been diversified from an aluminum plate formed of substantially pure aluminum to an aluminum plate in which manganese is added to increase the strength of the aluminum plate corresponding to different user applications.
In order to use an aluminum plate as a planographic printing plate support, it is necessary that the aluminum plate have a proper adhesion property to a photosensitive material and a proper water-retention property.
To this end, it is necessary to make the surface of an aluminum plate rough so that the aluminum plate has a uniformly and finely grained surface. This surface-roughing treatment greatly affects the printing performance, such as anti-stain performance, of a plate material printing. Accordingly, the quality of the surface-roughing treatment has been an important factor in producing plate materials.
As a method of performing surface-roughing on an aluminum printing-plate support, it is possible to use one of a mechanical graining method, an electrochemical graining method, or to use those graining methods in combination.
As a mechanical graining method, there are, for example, a ball graining method, a wire graining method, a brush graining method, a liquid horning method, and the like. As an electrochemical graining method, on the other hand, an AC electrolytic etching method has been generally employed. In this case, an electric current of an ordinary sinusoidal waveform, or a special waveform, such as a square waveform, has been used. Further, as pre-treatment for the foregoing electrochemical graining, a chemical etching treatment or oil removing treatment with alkaline solution such as sodium hydroxide or sodium silicate may be performed.
In the AC electrolytic etching method among the foregoing methods, however, there has been a problem in that counter electrodes of carbon, metal, or the like are very easily deteriorated. For example, in the case of using counter electrodes of carbon, deterioration of a binder is significant because oxidation and reduction are repeated every time the polarity changes, and therefore it has been very difficult to perform a stable operation for a long time.
In order to solve the problems, Japanese Patent Examined Publication No. Sho. 61-48596 discloses an electrolytic treatment apparatus which is characterized in that a circuit for an auxiliary counter electrode is connected in parallel to a circuit connected to main counter electrodes, and a diode for controlling an anode current flowing in the main counter electrodes, or a mechanism functioning as such a diode, are provided in the circuit for the auxiliary counter electrode. The electrolytic treatment apparatus has such a configuration as shown in FIG. 2, in which a metal web 1, that is, a material to be treated, is supported on the circumference of a radial drum roller 2, and a clearance between the metal web 1 and each of the main counter electrodes 3a and 3b disposed in opposition to the metal web 1 is filled with an electrolytic treatment solution 5 containing metal ions in a manner so that the electrolytic treatment solution 5 is supplemented from an electrolyte supply inlet 4 and discharged from an electrolyte discharge outlet 6 to thereby form an electrolyte path 15. An AC current is supplied through electrolyte path 15 from an AC power source to the main counter electrodes 3a and 3b so as to perform electrochemical treatment. The counter electrodes opposed to the metal web 1 are constituted by the main counter electrodes 3a and 3b and the auxiliary counter electrode 8; a circuit for the auxiliary counter electrodes 8 and a circuit for making an anode current flow into the main counter electrodes are connected in parallel to a circuit connected to the main counter electrodes 3a and 3b, and a diode 9 for controlling the anode current flowing in the main counter electrodes or a mechanism functioning as such a diode is provided in the circuit for the auxiliary counter electrode 8 so that currents are made to flow in those circuits.
In this case, the main counter electrodes 3a and 3b are connected to opposite sides of the AC power source 7 so as to have polarities which are different from each other. Further, each of the main counter electrodes 3a and 3b is constituted by a large number of small electrodes (3a.sub.1, 3a.sub.2, 3a.sub.3, . . . , 3a.sub.n,), (3b.sub.1, 3b.sub.2, 3b.sub.3, . . . , 3b.sub.n,) (for example, n=10 to 24), each separated from adjacent ones through insulators 10 so as to raise the current efficiency.
In the conventional electrolytic treatment apparatus, however, there has been a problem in that the electrolytic treatment solution 5 supplemented from the electrolyte supply inlet 4 passes through the narrow electrolyte path 15 between the metal web 1 and each of the electrodes 3a and 3b, and flows into the opposite side of the radial drum roller 2 supply inlet 4 so as to come out the path 15 to the electrolyte discharge outlet 6. This result is that the electrolytic treatment solution gradually fatigues because of electrolysis in the flow path and a component difference is caused between the electrolytic treatment solution at the electrolyte supply inlet 4 and that at the electrolyte discharge outlet 6. This makes it impossible to obtain satisfactory electrolysis efficiency as the whole with the electrolytic treatment apparatus.
Further, a difference between the temperature at the electrolyte supply inlet 4 and that at the electrolyte discharge outlet 6 increases in the vicinity of the metal web so that it is impossible to obtain a desired grained surface.
In the electrolyte path 15, the treatment solution at the metal web 1 side is not sufficiently mixed with the treatment solution at the side of the counter electrodes 3a and 3b so that the difference in degree of fatigue of the component of the treatment solution between at the metal web side and the counter electrodes side, as well as the temperature difference therebetween, are significant. As a result, unevenness is caused in graining, that is, the electrolytic quality is lowered and the electrolysis efficiency is reduced.
In order to raise the electrolysis efficiency while maintaining uniform and fine grain, therefore, a method has been used in which the flow rate of the treatment solution supplied from the electrolyte supply inlet 4 is increased. In this method, however, the increase in supply of the treatment solution not only causes an increase in cost, but also brings about no sufficiently desirable grain even if the rate of flow of treatment solution is increased.
The foregoing problems become significant when the length of electrolytic treatment is prolonged corresponding to a rise of the line speed, so that there has been a significant limit in increasing the line speed.